The instant invention relates generally to a coating hood for applying a protective coating to hollow glass containers, and more particularly, to an air flow guide which directs streams of vapor free air over the finish of the containers to limit the accumulation of coating compound thereon.
The desirability of applying protective coatings to the exterior of hollow glass containers has long been recognized. Such coatings, which include tin, titanium, or other heat decomposable metallic compounds, or organometallic compounds, protect the glass containers from surface damage, such as abrasions and scratches, which result in a loss of tensile strength for the glass containers. The need for high-tensile strength in glass containers is particularly acute when containers are massproduced, move rapidly in close proximity along high-speed conveyor lines, and are subsequently filled with carbonated beverages, beer, wine, foodstuffs, and the like that produce gaseous pressure within the container.
Such protective coatings are usually applied as the glass containers emerge in a heated, fully-shaped condition from a glassware forming machine, that is, at the "hot end" of the system. The containers are transported away from the forming machine by a conveyor. Temperatures in excess of 400 degrees Centigrade exist at the surface of the glass containers, so that when a heat decomposable inorganic metallic, or organometallic, compound is applied thereto, the compound decomposes immediately and is converted to a metallic oxide coating.
One well-known and previously widely used technique for applying a protective coating to the hot glass containers calls for spraying the opposite sides of the containers as they travel on a conveyor, in single file, through two longitudinally spaced, oppositely positioned spray heads. Each spray head covers approximately 180 degrees of the circumference of the bottle, so that at least two spraying stations are required to coat the entire bottle. Receivers are positioned at the opposite side of the conveyor in alignment with the spray heads. Pressurized air with the coating compound entrained therein is discharged from each spray head at a significant pressure, while the receivers are usually maintained at a low pressure. The net pressure differential increases the velocity, and thus the effectiveness, of the coating compound. Coating systems of this nature are disclosed in U.S. Pat. No. 3,516,811, granted June 23, 1980, to G. L. Gatchet et al., and U.S. Pat. No. 3,684,469, granted Aug. 14, 1972 to W. C. Goetzer et al.
Gatchet et al. recognized that the deposition of a metallic oxide coating on the finish of the glass container passing on a conveyor through the open-sided coating apparatus was undesirable, as noted in column 3, lines 21-57. To control the location of the metal oxide deposit, as well as the uniformity of the deposit, Gatchet et al. relied upon spray heads producing a (theoretically) laminar flow that would pass laterally across the width of the conveyor, as shown in FIG. 4 of the patent.
The above-described coating systems, however, are "open-sided" and are thus adversely influenced by ambient conditions in the factory where the glass containers are formed. The ambient conditions of prime concern are rapidly moving air currents, the moisture in the air, and the potentially toxic and/or corrosive fumes and pollutants issuing from the spray heads. Air currents can cause turbulent conditions at the spray heads that will cause a preferential or uneven, application of the protective coating, and some of the coating will accumulate on the bottle finish. The rapidly moving air currents disrupt the laminar flow patterns that are theoretically possible with "open-sided" systems, and the capability of uniformly, and consistently, applying the same thickness of coating is severly reduced. To compensate for air currents, the above-described systems are operated at higher pressures, and with greater throughput of coating compound, than would be required under quiescent conditions. The moisture in this hostile atmosphere causes hydrolysis loss by rendering some of the compound unfit for its intended purpose. The escape of potentially toxic fumes into the atmosphere at the work place constitutes an occupational health hazard that may violate Federal, state and local ordinances. Also, the toxic fumes are highly corrosive and attack various components of the glass factory, such as the blowers, exhaust systems, conveyors, and even roofs, leading to increased plant maintenance costs. Furthermore, the efficiency of these "open-sided" systems is low, since much of the relatively expensive coating compound is wasted.
A second, well-known, and widely employed technique for applying a protective coating to hot glass containers relies upon a formed, sheet metal coating hood with spray heads and associated receivers situated therein. The coating hood obviates many of the problems associated with the open-ended spray systems discussed above. For example, the hood isolates the glass containers to be coated from the ambient conditions and furnishes a controlled atmosphere that enhances the coating operations. The hood includes an exhaust system which captures most of the air entrained coating compound that does not adhere to the containers, thus reducing the problems of venting the system and minimizing the opportunity for the coating compound to attack building components. Also, such hood can significantly raise the coating efficiency of the systems, with attendant cost savings.
Representative coating hoods are disclosed in U.S. Pat. No. 3,819,404, granted June 25, 1974 to Addison B. Scholes and Joseph J. Kozlowski, in U.S. Pat. No. 3,933,457, granted Jan. 20, 1976 to Addison B. Scholes, and in U.S. Pat. No. 4,389,234, granted June 21, 1983 to Georg H. Lindner, one of the co-inventors of the present application.
However, when a "hot end" coating is applied to glass containers, it is usually necessary to avoid depositing metal oxide coating on the upper part of the container including the neck and the threads; such area is commonly known as the finish of the container. Coating on the finish interferes with the application of a screw cap, lid, crown or the like to the container after the container has been filled. Additionally, the metal oxide coating on the finish may interact with the dissimilar metal of the cap, lid, crown, and the like to produce an unsightly corrosion problem or result in a high torque necessary for cap removal. Such corrosion problem, with its visual blemishes, is particularly acute since the coated glass containers are usually employed for beers, wines, soft drinks and foodstuffs, and the ultimate consumers will not purchase products even appearing to be tainted.
Still further, apart from the cap related problems, it is sometimes required that no coating be deposited inside the containers, particularly in medical and pharmaceutical applications. This, however, requires a coating system which completely avoids contact between the coating compound and the bottle finish. This requirement, however, has not been met with conventional types of coating systems. Therefore, because it is difficult to coat the main body of the container without coating the finish, it has been generally accepted that a coating thickness on the finish can be one-third that on the main body.
The aforementioned Lindner patent relies upon the high velocity flow of air with entrained coating compound passing across the width of the coating chamber to coat the glass containers passing therethrough. As shown in FIGS. 6 and 7 of the Lindner patent, a baffle with specifically configured apertures and horizontally extending ribs produces a substantially even flow of air over the full height of the coating chamber. While the operation of such hood has been far superior to previously known coating hoods in almost every aspect of performance, coating compound is deposited on the finish of the containers passing through the hood. Thus, this hood is only suitable for coating containers on which finish protection is of no concern.
At least one other known coating hood has also recognized the desirability of minimizing, if not preventing, the deposition of coating compound on the finish of the containers being coated, but have not realized this objective. While these other coating hoods have not even approached the Lindner coating hood in performance, they are discussed hereinafter to present a complete picture of the state of the art prior to the instant invention.
Another known coating hood is shown in U.S. Pat. No. Re. 28,076, granted July 16, 1974 to B. 0. Augustsson and R. S. Southwick. FIG. 2 of the Augustsson et al. patent, which is assigned to the Glass Container Manufacturers Institute, Inc. New York, N.Y., shows a coating hood employing a horizontal barrier in each chamber in both side walls to divide each coating chamber into an upper chamber and a lower chamber of approximately equal dimensions. A stream of air with coating compound entrained therein is discharged from jet slots or nozzles in the lower chamber to coat the body of the glass bottle passing thereby; and the air stream with the unused coating compound passes into the lower chamber defined in the opposite side wall of the hood and is recirculated in a continuous manner.
A separate, distinct stream of fresh air, free from the vapor of the coating compound, is blown through the upper chamber in one side wall and is exhausted through the upper chamber in the opposing side wall, and through the top of the hood in a single pass. Both streams of air pass across the lateral extent of the coating hood in the same direction, and at high velocities. The upper stream of fresh air is intended to keep the coating compound in the lower stream of air from reaching the container finish. However, under the hostile operating conditions experienced in glass manufacturing plants, the two streams of air intermix to a certain extent and have proven incapable of consistently keeping the coating compound removed from the container finish. It was found in actual tests that there was a coating thickness of approximately 12 CTUs (coating thickness units) on the finish with this hood. A CTU is a well known measure of the thickness of the coating by the American Glass Research Institute of Butler, Pa. This corresponded to a thickness on the finish of approximately 30% of the coating thickness on the main body of the container. Even employing higher flow velocities in the upper vapor free air stream failed to consistently keep the coating compound from being deposited on the finish of the container. Such increased flow velocities also provide an increase in the amount of fresh air that is used, which results in an excessive usage of coating compound. In addition, only part of the hood length, in the direction of travel of the containers, is provided with the fresh air streams for finish protection, further increasing the amount of coating compound deposited on the finish of the containers.
In addition to the aforementioned coating hoods shown in the patents to Augustsson et al and Lindner, the corporate assignee of Lindner has attempted providing the Lindner hood with an inverted U-shape to allow the neck of the bottle to pass therethrough. Fresh air was blown from rows of holes in the sides of the inverted U-shaped roof to the neck of the bottle, as schematically shown in FIG. 5. In such hood, however, the two streams intermix and cause a marked decrease in the deposition rate of the coating compound. It was found that, with such hood, the coating on the finish had a thickness generally one-third that of the thickness of the coating on the main body of the container, which is clearly undesirable. This was due to the finish air mixing with the coating air, whereby the coating compound was applied to the finish of the containers. Further, due to the dilution of the coating air with the finish air, the consumption of coating compound increased by as much as 20% to maintain the required coating thickness on the main body of the container.
The foregoing coating hoods, particularly the Lindner coating hood, performed satisfactorily under most operating conditions, but did not realize the desirable objective of minimizing, if not completely avoiding, the deposition of coating compound on the finish of the containers being coated. Such objective, if achieved, would eliminate the amount of coating on the finish without increasing the compound used per container. Furthermore, the absence of coating compound on the finish of each container avoids unsightly corrosion problems when caps are applied to the containers, while the strength of the container is retained undiminished. Also, if such objective could be achieved without using an inordinate number of blowers to force the air across the coating chamber, a relatively inexpensive and easy to operate coating hood could be realized.